Electrical power systems

ABSTRACT

Electrical power systems for distributing electrical power in arrangements including one or more gas turbine engines; one such system includes: one or more rotary electric machines, coupled to a gas turbine engine spool; converter circuits connected to the rotary electric machines converting between alternating current (ac) and direct current (dc), wherein the rotary electric machines and the converter circuits output a number R≥2 of dc power channels, each channel having an index r=(1, . . . , R); and a group of N dc load channels connected to the R dc power channels by a switching arrangement, wherein each dc load channel has an index n=(1, . . . , N) and R&lt;N≤2R. The switching arrangement operates to connect the r-th power channel to the n-th load channel according to the relationship: 
     
       
         
           
             
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CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority fromUnited Kingdom Patent Application Number 2102977.2, filed on 3 Mar.2021, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure concerns electrical power systems for distributingelectrical power, particularly in arrangements comprising rotaryelectric machines connected to gas turbine engine spools, for examplegas turbine engines of aircraft.

BACKGROUND

In aerospace, the more electric engine (MEE) and more electric aircraft(MEA) concepts have been advocated for the substantial reduction in fuelconsumption and complexity they bring. For example, one known aircraftconfiguration includes in its engines electric machines operable as bothmotors and generators so as to facilitate both generation of electricalpower during flight but also starting of the engine and removal of theair-turbine starter. One engine configuration for this known aircraftincludes such electric machines coupled to the high-pressure spool of atwin-spool turbofan. Another includes such electric machines coupled tothe intermediate-pressure spool of a triple-spool turbofan. Someconfigurations facilitate power transfer between spools of a gas turbineengine, which studies have shown can provide improvements in terms ofcomponent life and specific fuel consumption as well as a reduction inthe size of the turbomachinery.

Furthermore, the desire to reduce fuel consumption and emissions hascreated increased interest in hybrid electric (HE) aircraft. Serieshybrid architectures include electrically-driven propulsors, with theelectrical power being provided by a combination of a generator, whichis driven by a gas turbine or other rotating machinery, and batteries,with the latter generally be charged by the former during some flightstages. Parallel hybrid architectures typically utilise a turbofan orturboprop engine for propulsion, with batteries which drive electricmotors coupled to the spools of the turbomachinery providing additionaldrive power to the engine. Other hybrid electric propulsion systemsshare features of both the series and parallel architectures. Purelyelectric (PE) aircraft, whose power and propulsion systems includeelectric machines and batteries and/or fuel cells, are also of interest.

Previous designs have, typically, delivered power to engine accessoriessuch as the engine fuel pump via a mechanical interface between anengine accessory gearbox, which is connected to an engine spool, and anelectrical generator dedicated to powering the fuel pump. More electricand hybrid electric platforms may however make greater use of electricaldistribution systems that are not dedicated to a particular function.For instance, electrically-powered engine accessories and non-engineaircraft electrical systems may both be powered using electrical powertaken from a non-dedicated electrical power system.

Whilst the latter approach may increase efficiency and reduce weight andcomplexity, it has the potential to create hazardous new faultconditions and thus system certification issues. Service experience hasshown that the means of electrical generation and distribution in suchconfigurations can and do fail from time to time. It is essential thatif a fault occurs in the electrical generation and/or distributionsystem, critical electrical loads such as an electric fuel pump canstill function. Further, if an electrical load experiences a fault thenthat fault should not affect the functionality of the wider electricaldistribution system and the other electrical loads. For instance, in amore electric aircraft, if there is a fault affecting one electricalload, the fault should not affect the function of other loads,especially critical loads such as fuel and oil pumps. In ahybrid-electric aircraft, a fault in a non-propulsive load should not becapable of starving the propulsion system of electrical power, norshould the propulsion system starve critical non-propulsive loads incase of a fault or transient event.

Thus, it is an object of the invention to provide an electrical powersystem for distributing electrical power in a suitably fault-tolerantmanner.

In this specification, the following mathematical notation is assignedto objects for the purposes of clarity and conciseness. A collection ofobjects having an ordered relationship may be represented as a sequenceof members. For a finite sequence σ of length N, each member has anassociated index n identifying its position in the sequence.

As used herein, the length of a sequence is denoted by an uppercaseletter, with the variable representing the indices of the members of thesequence denoted by the equivalent lowercase letter. In this way it ispossible herein to concisely define properties of each member of asequence.

For example, let a sequence α have length N=8 such that the members eachhave an associated index n. It is possible to state that for all n≡0(mod 2), the nth member has property X. In this way a particularproperty is defined over the even-indexed members. This is in contrastto having to set out that each of the second member, the fourth member,the sixth member, and the eighth member has property X. Thus it may beseen that great improvements in conciseness are possible for sequencesof large length with properties shared over a mathematically-definedsub-sequence.

Furthermore, say we wish to describe links between the members of amultiplicity of sequences. For example, let a sequence β have a lengthP=3, and let a sequence γ also have a length P=3. The members of bothsequences therefore have an index p associated therewith. Given thesedefinitions, we may concisely state that for all p=(1 . . . P), the pthmember of sequence β is linked to the pth member of sequence γ. This isin contrast to having to set out that the first member of sequence β islinked to the first member of sequence γ, the second member of sequenceβ is linked to the second member of set γ, and the third member ofsequence β is linked to the third member of sequence γ. Whilst thestatements are equivalent, it will be appreciated that by using thenotation explained heretofore significant improvements in concisenessare achieved.

SUMMARY

The invention is directed towards electrical power systems fordistributing electrical power in arrangements in which electric machinesare connected with rotating machinery, which rotating machinery maycomprise, for example, gas turbine spools. Thus, the invention is alsodirected towards a gas turbine comprising such electrical systems,arrangements comprising two gas turbines and such electrical systems,propulsion systems, for example hybrid electric propulsions systems,comprising one or more gas turbines and such electrical systems, andaircraft comprising the aforementioned systems and arrangements.

According to a first aspect, there is provided an electrical powersystem for distributing electrical power in an arrangement comprisingone or more gas turbine engines. The electrical power system comprises:one or more rotary electric machines, each mechanically coupled to a gasturbine engine spool; a set of converter circuits connected to the oneor more rotary electric machines for conversion between alternatingcurrent (ac) and direct current (dc), wherein the one or more rotaryelectric machines and the set of converter circuits are arranged tooutput a number R≥2 of dc power channels, each dc power channel having arespective index r=(1, . . . , R); and a group of N dc load channelsconnected to the R dc power channels by a switching arrangement, whereinN>R and each dc load channel has a respective index n=(1, . . . , N).

The following features may be applied singularly or in combination witheach other and all of the aspects provided herein.

The rotary electric machines may be directly coupled to the gas turbineengine spools or indirectly coupled to the gas turbine engine spools,for example via an engine accessory gearbox.

The switching arrangement may comprise switching elements, for examplecontactors or circuit-breakers. The switching elements may besolid-state switching elements, electromechanical switching elements orhybrid solid-state and electromechanical switching elements. The openingand closing of the switching elements may be controlled in order tocommutate the electrical load current.

The system may further comprise, for each respective one of a pluralityof the N load channels, a current limiting device (CLD) operable tolimit an amount of current flowing from the power channels to a loadconnectable to the electrical power system via the respective loadchannel.

The system may comprise a CLD for each one of the N load channels

Each respective CLD may instead be operable to limit an amount ofcurrent flowing to the power channels from the load.

Each respective CLD may be bidirectional such that it is furtheroperable to limit an amount of current flowing to the power channelsfrom the load. Each bidirectional CLD may include a pair ofunidirectional CLDs connected in anti-parallel.

Each CLD may comprise one or more of: a passive CLD component such as aninductor; an active CLD component such a current limiting diode; and/ora superconducting fault current limiter.

The CLDs may be: located on a side of the switching arrangement oppositeto the power channels; or located between the power channels and theswitching arrangement; or incorporated into the switching elements ofthe switching arrangement.

The switching arrangement may be operable to connect a number Q≥1 of theN load channels to at least two different power channels of the R powerchannels.

The system may further comprise a controller configured to control theoperation of the switching arrangement to control which of the powerchannels the N load channels receive electrical power from.

The controller may be configured to control the switching arrangement sothat, at all times, each respective one of the N load channels receivespower from only one of the power channels to which the switchingarrangement can connect it.

The controller may be configured to control the switching arrangement sothat in a no fault condition, each one of the R power channels deliverspower to a number of load channels not exceeding ┌N/R┐.

In a no fault condition, each one of the Q load channels may beconnected to and receive power from both of the two different powerchannels to which it is connected by the switching arrangement. Forexample, switching elements (e.g. dc contactors) associated with both ofthe power channels may be closed so that the load channel is connectedto and receives power from both of the power channels.

The controller may be configured to control the operation of theswitching arrangement so that in a fault condition in which any one ofthe R power channels has a fault, each one of the R power channelsdelivers power to a number of load channels not exceeding ┌N/R┐+1.

The controller may be configured to control the operation of theswitching arrangement so that, in a fault condition in which one of theR power channels has a fault, each one of the respective Q load channelsreceives power from a non-faulted power channel of the at least two dcpower channels to which the respective one of the Q loads channel can beconnected by the switching arrangement.

In some embodiments Q=N, such that the switching arrangement is operableto connect every one of the N load channels to at least two differentpower channels.

In some embodiments Q<N, and the switching arrangement is operable toconnect the remaining N−Q load channels to only one of the R powerchannels.

In some embodiments Q=N−2, and the switching arrangement is operable toconnect the n=1 load channel to only the r=1 power channel; the n=N loadchannel to only the r=R power channel; and, for each respective one ofthe remaining Q=N−2 load channels, the respective load channel to atleast two different power channels of the R power channels.

In some embodiments Q=N−R, and the switching arrangement connects then=(1, . . . , R) load channel to only the r=(1, . . . , R) powerchannel; and the switching arrangement is operable to connect each ofthe remaining Q=N−R load channels to at least two different powerchannels.

In some embodiments, for each respective one of the number Q of the Nload channels, the switching arrangement is operable to connect therespective load channel to precisely two different power channels.

In some embodiments, R<N≤2R. In these embodiments, the switchingarrangement may be operable to connect the r-th power channel to then-th load channel according to the relationship:

${r(n)} = \left\{ \begin{matrix}{1,} & {n = 1} \\{{{\frac{n}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n}{2}} + 1},} & {{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {0\ \left( {{mod}2} \right)}} \\{{\frac{n - 1}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n + 1}{2}},} & {{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {1\ \left( {{mod}2} \right)}} \\{R,} & {n = {N.}}\end{matrix} \right.$

In some embodiments, N=2R.

In some embodiments, R=2, 3 or 4.

In some embodiments, N≥4.

The N dc load channels may be connected to and power a poly-phaseelectrical load. A controller may be configured to control the switchingarrangement so that, in a no fault condition, pairs of phases of thepoly-phase electrical load are powered by the same power channels so asto reduce the harmonic content in the supply current. The two phases ofeach pair of phases may be electrically separated by 360/P degrees, Pbeing a total number of phases of the poly-phase electrical load.

The electrical power system may further comprise a set of convertercircuits for conversion between direct current flowing in the loadchannels and alternating current for powering one or more ac loads.

The N load channels may be connected to one or more loads including atleast one of: an electrically driven fuel pump; an electrically drivenoil pump; and an electrically driven cabin blower.

The N load channels may be connected to one or more loads including atleast one propulsive load. The propulsive load may comprise anelectrical machine comprising at least two submachines.

According to a second aspect, there is provided a gas turbine enginecomprising the electrical power system of the first aspect. The one ormore rotary electric machines of the electrical power system may bemechanically coupled to one or more spools of the gas turbine engine.

According to a third aspect, there is provided arrangement comprising: afirst gas turbine engine having a spool; a second gas turbine enginehaving a spool; and the electrical power system of the first aspect. Theone or more rotary electric machines of the electrical power system mayinclude a first rotary electric machine mechanically coupled to thespool of the first gas turbine engine and a second rotary electricmachine connected to the spool of the second gas turbine engine.

According to a fourth aspect, there is provided an aircraft propulsionsystem comprising the electrical power system of the first aspect. Thepropulsion system may be a hybrid electric propulsion system. Thepropulsion system may comprise: one or more gas turbine engines, whereinthe one or more rotary electric machines of the electrical power systemare mechanically coupled to one or more spools of the one or more gasturbine engines; and one or more propulsors. The R power channels of theelectrical power system may be arranged to deliver electrical power tothe one or more propulsors, and the N load channels may be electricallyconnected to one or more non-propulsive loads.

According to a fifth aspect, there is provided an electrical powersystem for distributing electrical power in an aircraft, the electricalpower system comprising: an electrical power source configured to outputa number R≥2 of dc power channels, each dc power channel having arespective index r=(1, . . . , R); and a group of N dc load channelsconnected to the R dc power channels by a switching arrangement, whereineach dc load channel has a respective index n=(1, . . . , N) and R<N≤2R;wherein the switching arrangement is operable to connect the r-th powerchannel to the n-th load channel according to the relationship:

${r(n)} = \left\{ \begin{matrix}{1,} & {n = 1} \\{{{\frac{n}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n}{2}} + 1},} & {{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {0\ \left( {{mod}2} \right)}} \\{{\frac{n - 1}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n + 1}{2}},} & {{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {1\ \left( {{mod}2} \right)}} \\{R,} & {n = {N.}}\end{matrix} \right.$

The electrical power source may comprise an energy storage system, forexample one or more batteries e.g. one or more battery packs or batterymodules). In some embodiments the electrical power source comprises oneor more fuel cells.

According to a sixth aspect, there is provided an aircraft comprising anelectrical power system of the first aspect, a gas turbine engineaccording to the second aspect, an arrangement according to the thirdaspect, a propulsion system according to the fourth aspect or anelectrical power system according to the fifth aspect.

The skilled person will appreciate that except where mutually exclusive,a feature described in relation to any one of the above aspects may beapplied mutatis mutandis to any other aspect. Furthermore except wheremutually exclusive any feature described herein may be applied to anyaspect and/or combined with any other feature described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and not toscale, and in which:

FIG. 1 shows a general arrangement of a turbofan engine for an aircraft,including a rotary electric machine on each spool thereof;

FIG. 2A shows an exemplary arrangement for connecting two rotaryelectric machines to two dc power channels;

FIG. 2B shows the winding configuration of one of the electric machinesof FIG. 2A;

FIG. 3A shows an exemplary arrangement for connecting a single rotaryelectric machine to two dc power channels;

FIG. 3B shows the winding configuration of the electric machine of FIG.3A;

FIG. 4 shows a switching arrangement for connecting a group of N=4 dcload channels to R=2 dc power channels;

FIG. 5 shows a switching arrangement for connecting a group of N=6 dcload channels to R=3 dc power channels;

FIG. 6 shows a switching arrangement for connecting a group of N=8 dcload channels to R=4 dc power channels;

FIG. 7 shows a switching arrangement for connecting a group of N=3 dcload channels to R=2 dc power channels;

FIG. 8 shows an alternative switching arrangement for connecting a groupof N=6 dc load channels to R=3 dc power channels;

FIG. 9 shows a further alternative switching arrangement for connectinga group of N=6 dc load channels to R=3 dc power channels;

FIG. 10 shows still a further alternative switching arrangement forconnecting a group of N=6 dc load channels to R=3 dc power channels;

FIG. 11 shows a switching arrangement for connecting a group of N=8 dcload channels to R=3 dc power channels;

FIG. 12 shows a further switching arrangement for connecting a group ofN=8 dc load channels to R=4 dc power channels;

FIG. 13A shows combinations of a switch and a current limiting device;

FIG. 13B shows an exemplary current limiting device in the form of acurrent limiting diode;

FIG. 14A shows how a current limiting device responds to a downstreamelectrical overload and how this affects the current drawn from thepower channels and the voltage across other loads; and

FIG. 14B shows how a current limiting device responds to a downstreamtransient over-voltage event and how this affects the voltage acrossother loads.

DETAILED DESCRIPTION FIG. 1

A general arrangement of an engine 101 for an aircraft is shown inFIG. 1. In the present embodiment, the engine 101 is of turbofanconfiguration, and thus comprises a ducted fan 102 that receives intakeair A and generates two pressurised airflows: a bypass flow B whichpasses axially through a bypass duct 103 and a core flow C which entersa core gas turbine.

The core gas turbine comprises, in axial flow series, a low-pressurecompressor 104, a high-pressure compressor 105, a combustor 106, ahigh-pressure turbine 107, and a low-pressure turbine 108.

In operation, the core flow C is compressed by the low-pressurecompressor 104 and is then directed into the high-pressure compressor105 where further compression takes place. The compressed air exhaustedfrom the high-pressure compressor 105 is directed into the combustor 106where it is mixed with fuel and the mixture is combusted. The resultanthot combustion products then expand through, and thereby drive, thehigh-pressure turbine 107 and in turn the low-pressure turbine 108before being exhausted to provide a small proportion of the overallthrust.

The high-pressure turbine 107 drives the high-pressure compressor 105via an interconnecting shaft. The low-pressure turbine 108 drives thelow-pressure compressor 104 via another interconnecting shaft. Together,the high-pressure compressor 105, high-pressure turbine 107, andassociated interconnecting shaft form part of a high-pressure spool ofthe engine 101. Similarly, the low-pressure compressor 104, low-pressureturbine 108, and associated interconnecting shaft form part of alow-pressure spool of the engine 101. Such nomenclature will be familiarto those skilled in the art.

The fan 102 is driven by the low-pressure turbine 108 via a reductiongearbox in the form of a planetary-configuration epicyclic gearbox 109.Thus in this configuration, the low-pressure turbine 108 is connectedwith a sun gear of the gearbox 109. The sun gear is meshed with aplurality of planet gears located in a rotating carrier, which planetgears are in turn are meshed with a static ring gear. The rotatingcarrier drives the fan 102 via a fan shaft 110. It will be appreciatedthat in alternative embodiments a star-configuration epicyclic gearbox(in which the planet carrier is static and the ring gear rotates andprovides the output) may be used instead.

As described previously, it is desirable to facilitate a greater degreeof electrical functionality on the airframe and on the engine. To thisend, the engine 101 comprises one or more rotary electric machines,generally capable of operating both as a motor and as a generator. Thenumber and arrangement of the rotary electric machines will depend tosome extent on the desired functionality. Some embodiments of the engine101 include a single rotary electric machine 111 driven by thehigh-pressure spool, for example by a core-mounted accessory drive 112of conventional configuration. Such a configuration facilitates thegeneration of electrical power for the engine and the aircraft and thedriving of the high-pressure spool to facilitate starting of the enginein place of an air turbine starter. Other embodiments, including the oneshown in FIG. 1, comprise both a first rotary electric machine 111coupled with the high pressure spool and a second rotary electricmachine 113 coupled with the low pressure spool. In addition togenerating electrical power and the starting the engine 101, having bothfirst and second rotary machines 111, 113, connected by powerelectronics, can facilitate the transfer of mechanical power between thehigh and lower pressure spools to improve operability, fuel consumptionetc.

As mentioned above, in FIG. 1 the first rotary electric machine 111 isdriven by the high-pressure spool by a core-mounted accessory drive 112of conventional configuration. In alternative embodiments, the firstelectric machine 111 may be mounted coaxially with the turbomachinery inthe engine 101. For example, the first electric machine 111 may bemounted axially in line with the duct between the low- and high-pressurecompressors 104 and 105. In FIG. 1, the second electric machine 113 ismounted in the tail cone 114 of the engine 101 coaxially with theturbomachinery and is coupled to the low-pressure turbine 108. Inalternative embodiments, the second rotary electric machine 113 may belocated axially in line with low-pressure compressor 104, which mayadopt a bladed disc or bladed drum configuration to provide space forthe second rotary electric machine 113. It will of course be appreciatedby those skilled in the art that any other suitable location for thefirst and (if present) second electric machines may be adopted.

In the present embodiment, the first and second electric machines 111,113 are connected with power electronics. Extraction of power from orapplication of power to the electric machines is performed by a powerelectronics module (PEM) 115. In the present embodiment, the PEM 115 ismounted on the fan case 116 of the engine 101, but it will beappreciated that it may be mounted elsewhere such as on the core of thegas turbine, or in the vehicle to which the engine 101 is attached, forexample.

Control of the PEM 115 and of the first and second electric machines 111and 113 is in the present example performed by an engine electroniccontroller (EEC) 117. In the present embodiment the EEC 117 is afull-authority digital engine controller (FADEC), the configuration ofwhich will be known and understood by those skilled in the art. Ittherefore controls all aspects of the engine 101, i.e. both of the coregas turbine and the first and second electric machines 111 and 113. Inthis way, the EEC 117 may holistically respond to both thrust demand andelectrical power demand.

In an implementation contemplated herein, the one or more rotaryelectric machines 111, 113 and the power electronics 115 are configuredto output to or receive electric power from two dc busses (also referredto herein as dc power channels)—a configuration contemplated for somefuture more electric aircraft platforms. An exemplary two-machine, twodc bus configuration, suitable for shaft power transfer in addition toelectrical generation and distribution, will be described with referenceto FIGS. 2A and 2B. An exemplary single machine configuration thatoutputs to or receives electric power from two dc busses will bedescribed with reference to FIGS. 3A and 3B.

It is to be understood that the present invention, which will bedescribed with reference to FIGS. 4 to 14, is not limited to theconfigurations described with reference to FIGS. 2A-B and 3A-B. Indeed,the present invention may be implemented in a system that includes one,two or more rotary electric machines. These one or more machines may becoupled to the same spool or different spools of a single gas turbineengine, or to spools of different gas turbine engines. Further, althoughan architecture with two dc busses is contemplated, the presentinvention may also be implemented in architectures having three, four ormore dc busses. The skilled person will appreciate that the arrangementsdescribed in FIGS. 2A-2B and 3A-3B, and indeed other arrangements, maybe modified to output to three or more dc busses according to thedesired application.

It should also be understood that the present invention may beimplemented within a hybrid electric architecture. For example, the oneor more rotary electric machines described above may take the form ofelectrical generators connected to a gas turbine engine in a serieshybrid architecture, with the two or more dc busses powering propulsiveand non-propulsive loads. Similarly, the one or more rotary electricmachines described above may take the form of motor-generators connectedto propulsive turbomachinery in a parallel hybrid architecture. Thepresent invention may be implemented within a purely electricarchitecture, with one or more dc power sources such as batteries and/orfuel cells outputting dc busses in place of the spool-coupled rotaryelectric machines.

FIGS. 2A-2B

An electric drive system 201 for connecting first and second electricmachines 111 and 113 to high- and low-pressure spools of a gas turbineengine is shown in FIG. 2A. The electric drive system 201 is shown inthe form of a single line diagram, the conventions of which will befamiliar to those skilled in the art. Thus for alternating current (ac)a single line replaces a plurality of polyphase lines, and for directcurrent (dc) a single line replaces the +V and −V lines.

In the illustrated configuration, the ac output of the electric machinesis provided to the PEM 115. The configuration of the electric machineswill be described further with reference to FIG. 2B.

A controller 202 for the PEM 115 is provided in the EEC 117. In thepresent example, the controller 202 is a functional module implementedin software running on the EEC 117. It will be appreciated that inalternative embodiments the controller 202 may be implemented inhardware in the EEC 117. It will also be appreciated that the controller202 may be a separate module in addition to the EEC 117.

As shown in FIG. 2A, a first set of bidirectional converter circuits 203is connected with the first electric machine 111, and a second set ofbidirectional converter circuits 204 is connected with the secondelectric machine 113. The controller 202 is configured to control theoperation of the first and second sets of bidirectional convertercircuits 203 and 204 so as to control the operation of the electricmachines 111 and 113.

The bidirectional converter circuits are configured to convertalternating current to and from direct current. In the presentembodiment, the direct current output of the converter circuits isprovided to a switching arrangement 205 for connection to a two-channelaircraft dc network, comprising a first dc bus 206 and a second dc bus207.

In the present example, the switching circuit 205 is operable to connector disconnect the bidirectional converters to each other, and the dcbusses. In this way, various faults may be managed, whilst maintainingthe capability to transfer power between the gas turbine spools.

The arrangement and control of the switching arrangement 205 isdescribed in more detail in UK Patent Application Number 1913016.0 andcorresponding European Patent Application Number 20190181.6 (publishedas EP 3793051 A1), both of which are assigned to the present applicant.The entire contents of both applications are incorporated herein byreference.

The first electric machine 111 of FIG. 2A is shown in FIG. 2B. In thepresent embodiment, the configuration—so far as described herein—of thesecond electric machine 113 is the same.

The first electric machine 111 is a dual-wound rotary electric machine.The term “dual-wound” will be understood by those skilled in the art tomean that it may be considered to comprise two functionally separatesubmachines. Further, in the present implementation, these submachinesare three-phase submachines. It will be appreciated that the number ofphases could differ, and in particular may be greater than two. Themaximum number is typically limited by space constraints, and wouldnormally be less than nine.

As shown in the Figure, in the present embodiment the first electricmachine 111 comprises a stator 301 having twelve teeth 302. Six coils303, 304, 305, 306, 307, and 308 are wound on alternate teeth such thatthere is only one coil side per slot. This will be recognised by thoseskilled in the art as a concentrated winding arrangement. In this way,the coils are electrically, electromagnetically, thermally, andmechanically separated to provide fault-tolerance. In the presentembodiment, the “dual-winding” of the first electric machine 111 isachieved by designating opposite coils as part of separate submachines.Thus, coil 303 forms a phase ϕU1, and coil 306 forms a phase ϕU2. Asimilar configuration is adopted for the other coils. Phases ϕU1, ϕV1,and ϕW1 which form a first submachine are in the present embodimentconnected in a star winding (also known as a wye winding), as are—in aseparate connection—phases ϕU2, ϕV2 and ϕW2 which form the secondsubmachine. It will be appreciated that a delta winding may be usedinstead.

In the present embodiment, the first electric machine 111 is a permanentmagnet electric machine. Thus, in operation as a motor, magnetic fieldsgenerated by the coils 303 to 308 interact with permanent magnets on arotor 309 which generates torque. In operation as a generator, themagnetic field of the rotor 309 interacts with the coils 303 to 308 togenerate a voltage.

It will be appreciated that different machine types may be used. Forexample, a distributed winding scheme could be used. Also, machines ofinduction, wound-field or switched-reluctance type may be used. Further,the machines may instead be of transverse- or axial-flux configuration.

It is also contemplated that in alternative embodiments the submachinesmay be formed not by different winding sets wound around the same stator301, but by axially separate machines each having one of the twowindings of the dual winding. Alternatively, the machines may be ofradially-segmented stator type in which each submachine occupies adifferent sector of the stator.

It should also be appreciated that whilst the electric drive system 201of FIG. 2A includes two dual-wound machines 111, 113 that output, viathe sets of converter circuits 203, 204 and switching arrangement 205,two dc busses 206, 207, the arrangement could be generalised to agreater number of dc busses. For instance, an additional dual-woundmachine, along with an additional set of converters and additionalswitching could be used to provide a fault-tolerant drive system 201that outputs four dc busses.

FIGS. 3A-3B

As described previously, in an aspect a single motor-generator 111 maybe configured such that it outputs to or receives electrical power fromtwo dc busses 206, 207 in a more electric or hybrid electric aircraftinstallation. Such a system is shown in FIG. 3A, again in the form of alogical single line diagram.

The motor-generator 111 comprises four phase connections, ϕA, ϕB, ϕC,and ϕD, which are each connected with an ac side of a respectiveindependent phase drive circuit 203 a, 203 b, 203 c, and 203 d. As willbe described with reference to FIG. 3B, in the present embodiment theinternal topology of the motor-generator 111 is of duplex four phaseconfiguration, and thus in practice an additional set of connections maybe provided to provide parallel connection. Alternatively connection maybe made in series either internally to the motor-generator 111 orexternally.

Referring again to FIG. 3A, in the present embodiment the phase drivecircuits 203 a-d are bidirectional converter circuits. In a specificembodiment, the phase drive circuits 203 a-d are H-bridges accompaniedby appropriate filters, although it will be appreciated that any othersuitable bidirectional converter topology may be used, such as a neutralpoint clamped converter topology.

The phase drive circuits 203 a-d operate under control of a controller202, which co-ordinates the operation of the phase drive circuits 203a-d to effect rectification or inversion as appropriate.

In the present embodiment, the controller 202 in turn operates undercontrol of the EEC 117 in either a motor or generator mode in the knownmanner.

The dc sides of the phase drive circuits 203 a-d are connected to both afirst dc bus 206 and a second dc bus 207. A switching arrangement 205,comprising a set of eight electrical contactors (not shown in FIG. 3A),is connected between the dc side of the phase drive circuits 203 a-d andthe first and second dc busses 206, 207, providing reconfigurableconnection and isolation therebetween.

In the present configuration, the contactors of the switchingarrangement 205 operate under the control of controller 202. In anon-faulted mode of operation, phases ϕA and ϕC of the motor-generator111 are connected to the first dc bus 206, and phases ϕB and ϕD of themotor-generator 111 are connected to the second dc bus 207. This may beachieved by controller 205 setting four of the contactors to a closedcondition, and setting the other four contactors to an open condition.

This configuration of the system, which is described in more detail inUK Patent Application Number 1913080.6 (published as GB 2587186 A) andcorresponding European Patent Application Number 20192313.3 (publishedas EP 3793071 A), permits each pair of phases ϕA, ϕC and ϕB, ϕD toremain isolated during fault-free operation. Both of the aforementionedapplications are assigned to the present applicant and the entirecontents of both are incorporated herein by reference.

The winding configuration of the motor-generator 111 is shown inschematic form in FIG. 3B.

The motor-generator 111 comprises a rotor 310 located interior to astator 311. The rotor 310 is of permanent magnet configuration, and inthis example has 14 permanent magnet poles 312, i.e. the pole pairnumber p=7.

The stator 311 is configured as an alternate-wound stator, and in thisexample has sixteen teeth defining sixteen slots, i.e. the slot numberN_(S)=16. Eight evenly-spaced coils 313 to 320 are located on alternateteeth such that there is one coil side per slot—this arrangement mayalso be referred to as a modular winding. This provides physical,thermal, electrical, and magnetic isolation between the coils whichprovides fault tolerance. In a specific embodiment, the coils 313 to 320are configured as precision coils, i.e. coils formed identically suchthat each turn occupies a specific, predefined location on the coil.

Forming part of the electric drive system of FIG. 3A, themotor-generator 111 is configured as a four-phase machine. As notedpreviously, the stator 311 comprises eight coils 313 to 320. In thepresent embodiment, the motor-generator is configured as a duplextwo-phase machine, in which coil pairs separated by 180 degrees formpart of the same phase. The individual coils in the present example areconnected in parallel with the respective phase drive circuit, althoughit is envisaged that they may also be connected in series. The choicemay be made upon, for example, the ability to run cables and/or theinstallation space envelope for terminations, etc.

Thus, in the present example, coils 313 and 317 form a coil pairseparated by 180 degrees. Both coils form part of phase ϕA, with coil313 being labelled ϕA1, and coil 317 being labelled ϕA2. Similar angularseparation, nomenclature and labelling applies to the other coils. Thus,it may be seen that phase ϕB is mechanically separated by +45 degreesfrom ϕA, phase ϕC is separated by +90 degrees from ϕA, and phase ϕD isseparated by +135 degrees from ϕA. This approach of pairing coilstogether means that the machine is mechanically balanced, in particularduring a fault condition when one or more phases may be disabled.

It will be understood that the electric drive system of FIG. 3A with theduplex machine 111 that, together with the phase drive circuits 203 a-dand switching arrangement 205, outputs two dc power channels 206, 207could be generalised to a higher number of output dc power channels. Forinstance, the machine 111 could be provided with an additional twophases, and a further two bidirectional converters and four contactorscould be provided, to give a drive system that outputs four dc powerchannels.

FIGS. 4-14

As discussed previously, it is expected that more electric, hybridelectric and purely electric aircraft will make greater use ofelectrically powered loads and will use non-dedicated electrical powersources to power said loads. For instance, engine accessories such asfuel pumps, oil pumps and cabin blowers will not only becomeincreasingly electric in their design, but electrical power will bedelivered to them using dc power channels such as those output in thearrangements described above with reference to FIGS. 2A-B and 3A-B. Thisis in contrast to many conventional designs in which engine accessories,if electrically powered at all, have their own dedicated electricgenerator mechanically connected to an engine accessory gearbox.

Whilst this approach may be associated with reduced weight and increasedefficiency, the need to deliver electrical power to poly-phaseelectrical loads in a fault-tolerant manner may undermine thesebenefits. For example, delivering power to the poly-phase loads in afault-tolerant manner will require that the installed power capacity isincreased beyond the nominal load in case of a fault or failure in theelectrical power system, and increased installed capacity generallyresults in increased weight. Arrangements described herein withreference to FIGS. 4-14 seek to address this and other problems.

The following nomenclature and notation will be used herein. Anelectrical power system includes a generator electrical drive system,preferably a multi-redundant (e.g. dual-redundant) system such as one ofthose described with reference to FIGS. 2A-B and 3A-B, which outputs anumber R≥2 of dc busses (i.e. power channels). Each of the R dc powerchannels has a unique index r, where r=(1, . . . , R). At a positionelectrically downstream of the electrical drive system, a group of N dcload channels are segregated from the dc power channels for powering oneor more electrical loads connected thereto. Each of the N dc loadchannels within a group of dc load channels has a unique index n, wheren=(1, . . . , N). The group of N dc load channels is connected to the Rdc power channels by a switching arrangement. Downstream of theswitching arrangement, the N dc load channels are connectable to one ormore electrical loads. Where the electrical load(s) are ac loads, someor all of the N dc load channels may be connected to the load(s) via aset of DC-AC converter circuits, which convert between the directcurrent carried by the dc load channels and poly-phase alternatingcurrent that powers the one or more loads.

An example for which R=2, N=4 is described in detail with reference toFIG. 4. Further N=2R examples are described for R=3 and R=4 withreference to FIGS. 5 and 6. An R<N<2R example utilising an equivalentswitching connection arrangement is described with reference to FIG. 7.FIG. 8-10 illustrate alternative connection arrangements, all using R=3,N=6 as an example. FIG. 11 illustrates a connection arrangement forN>2R, specifically R=3, N=8. Finally, to illustrate some of theadvantages associated with the arrangements of FIGS. 4-11, FIG. 12illustrates a further switching arrangement for R=4, N=8.

The switching arrangements include switching elements, which may be dccontactors, circuit breakers or the like. In the examples describedherein, each switching element (e.g. dc contactor 411 in FIG. 4) is adouble pole contactor in which each pole of the dc source is isolatedfrom the corresponding load channel. It will however be understood thata single pole switch may be used if desired and considered suitable forthe application.

The contactors of the switching arrangement are preferably capable ofproviding fast fault response and precise control in order to commutatethe electrical load current from one power source to another and tominimise the impact of the faults on the functionality of the load(s).Technologies capable of this include solid-state circuit breakers, andhybrid electromechanical and solid-state circuit breakers.

All of the switching arrangements described herein may incorporate, forsome and preferably each of the N dc load channels, current limitingfunctionality. This is described in more detail with reference to FIGS.13A-B and 14A-B.

In a system with multiple electrical loads, the electrical power systemmay include multiple groups of dc load channels connected to andsiphoning power from the R dc power channels. For example, a first groupconsisting of N₁ dc load channels may be connected to the R dc loadchannels and a second group of N₂ dc load channels may also be connectedto the R dc load channels at a position electrically downstream of thefirst group. The number N₁ of dc load channels in the first group may bethe same as or different from the number N₂ of dc load channels in thesecond group.

FIG. 4

FIG. 4 illustrates a portion of an electrical power system in which adual dc power source (i.e. R=2) is connected to a group 400 of N=4 dcload channels by a switching arrangement 410 to facilitate the provisionof electrical power to a four-phase (P=4) electrical load. In thisembodiment the load is a four-phase permanent magnet motor-generator(PMMG) 430 incorporated into an electric fuel pump of a gas turbineengine. It will be appreciated that other known types of electricalmachine, for example a switched reluctance machine, could replace thePMMG of FIG. 4.

The portion of the electrical system includes two dc power channels 206,207, which may also be referred to as dc busses. The power channels 206,207 are connected to and receive electrical power from an electricalgeneration system having one or more rotary electric machines, eachconnected to a spool of a gas turbine engine. The specific details ofthe electrical generation system are not relevant to the arrangement ofthe portion of the system shown in FIG. 4. It is sufficient to say that,in this embodiment, the electrical generation system outputs two dcpower channels 206, 207.

The two power channels 206, 207 are electrically connected to the fourdc load channels 401-404 via a switching arrangement 410. The switchingarrangement 410 includes a set of N=4 switch arrays, the switches ofwhich may be dc contactors, circuit breakers or the like. In total, theset of N=4 switch arrays includes six (2N−2=6) dc contactors 411-416.

The opening and closing of the contactors of the switching arrangement410 is controlled by a controller, which in the interests of clarity isnot shown in FIG. 4. The controller may be the same controller 202 usedto control the PEM 115 in FIGS. 2A and 3A, or it may be anotherfunctional module of the EEC 117. Said functional module may beimplemented in software running on the EEC 117 or in hardware in the EEC117. Other suitable implementations, for example local control withinthe PEM 115, will occur to those skilled in the art.

In this embodiment the four load channels output to a set 420 of N=4converter circuits 421-424, which may be unidirectional (dc to ac)converter circuits or bidirectional converter circuits. In oneembodiment the converter circuits 203 a-d are H-bridges accompanied byappropriate filters, although it will be appreciated that any othersuitable converter topology may be used.

The ac outputs of the converter circuits 421-424 connect to the fourphases A-D of the four-phase rotary electric machine 430. It is notedthat FIG. 4 only illustrates the connections between the ac outputs ofthe converters 421-424 and one phase coil A1-D1 per phase. In practiceconnections will also be made to the other phase coils A1′-D1′, ifpresent.

In this embodiment the four-phase rotary machine 430 drives an enginefuel pump. It is to be understood, however, that the four-phase machine430 could drive another engine accessory (such as an oil pump or cabinblower) or a non-engine electrical load.

In other embodiments, multiple loads may be connected to the output ofthe switching arrangement 410. In one group of embodiments, the fourconverter circuits 421-424 may be replaced with a set of converters thatincludes more than one converter circuit per dc load channel. Forexample, two four-phase rotary machines may be connected to theswitching arrangement 410 by utilizing two converter circuits connectedin parallel to each DC load channel. In still further embodiments, atwo- or three-phase load may be driven via two or three of the convertercircuits and the remaining one or two converter circuits may be omitted,with the remaining one or two dc load channels powering one or more dcloads either directly or via dc-dc converters.

Of particular note is the manner in which the switching arrangement 410connects the R=2 power channels 206-207 to the N=4 load channels401-404. This will now be described.

The n=1 load channel 401 is connected to the r=1 power channel 206 viathe first contactor 411 of the switching arrangement 410. Specifically,on one side of the switching arrangement 410, the first contactor 411 isconnected to the r=1 power channel 206 (by two connections consisting ofthe +V and −V lines) and, on the other side of the switching arrangement410, the first contactor 411 is connected to the n=1 load channel 401(again by two connections consisting of the +V and −V lines).

The n=N=4 load channel 404 is connected to the r=R=2 power channel 207via the sixth ((2N−2)-th) contactor 416. Specifically, the sixthcontactor 416 is connected to the r=2 power channel on one side and isconnected to the n=4 load channel 404 on the other side.

The n=2 load channel 402 is connected to the r=1 power channel 206 viathe second contactor 412 and is connected to the r=2 power channel 207via the third contactor 413. Specifically, on one side of the switchingarrangement 410, the second contactor 412 is connected to the +V and −Vlines of the r=1 power channel 206, whilst the third contactor 413 isconnected to the +V and −V lines of the r=2 power channel 207. On theother side of the switching arrangement 410, the +V lines of each of thesecond and third contactor 412, 413 are connected together to form a +Vline of the second load channel 402, whilst the −V lines of each of thesecond and third contactor 412, 413 are connected together to form a −Vline of the second load channel 402.

The n=3 load channel 403 is connected to the r=1 power channel 206 viathe fourth contactor 414 and is connected to the r=2 power channel 207via the fifth contactor 415. Specifically, on one side of the switchingarrangement 410, the fourth contactor 414 is connected to the +V and −Vlines of the r=1 power channel 206, whilst the fifth contactor 415 isconnected to the +V and −V lines of the r=2 power channel 207. Therespective +V lines of each of the fourth and fifth contactor pairs 414,415 are connected together to form a +V line of the third load channel403. The respective −V lines of each of the fourth and fifth contactorpairs 414, 415 are connected together to form a −V line of the thirdload channel 403.

Thus, the switching arrangement 410 connects the first load channel 401to the first power channel 206, connects the second load channel 402 toboth the first and second power channels 206, 207, connects the thirdload channel 403 to both the first and second power channels 206, 207,and connects the fourth load channel 404 to the first second powerchannel 207. This is summarised in the first and second columns of Table1:

TABLE 1 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 2 1 3 1 and 2 2 2 1 4 2 2 20 Max. Load Channels Per Power 2 3 3 Channel

The controller controls the opening and closing of the contactors of theswitching arrangement 410 such that each of the four load channels401-404 receives power from one of the two power channels 206, 207. Forthe n=1 load channel 401 this is necessarily the r=1 power channel 206.For the n=4 load channel 404 this is necessarily the r=2 power channel207. However for the n=2 and n=3 load channels 402, 403, one of the twopower channels 206, 207 is preferentially selected by controlling theopening and closing of the contactors 412-415. For instance, the n=2load channel 402 may be powered by the r=1 power channel 206 by closingthe second contactor 412 and opening the third contactor 413. The n=3load channel 403 may be powered by the r=2 power channel 207 by closingthe fifth contactor 415 and opening the fourth contactor 414.

Under normal operating conditions, with no faults in either of the twopower channels 206, 207, the controller controls the opening and closingof the contactors so that each of the two power channels 206, 207 powersprecisely two of the load channels 401-404. This is summarised in thirdcolumn of Table 1. Balancing the load channels between the power sourcesor power channels under normal operating conditions allows the system torun with reduced electrical and thermal stress.

During service it is possible that a fault condition will arise in whichone of the two power channels 206, 207 is disrupted. Total or partialloss of power in one of the two channels 206, 207 may occur. In thiscase, the controller is configured to control the opening and closing ofthe contactors of the switching arrangement 410 to reduce the impact onthe downstream load. Specifically, where a load channel was receivingpower from the now-faulted power channel and it is possible to connectthe load channel to another power channel, the controller opens thecontactor on the faulted channel and closes the contactor on the healthypower channel to provide power to the load channel.

The modified power sourcing in the case of a fault in the r=1 powerchannel 206 is summarised in the fourth column of Table 1. As can beseen, the loss of power in the r=1 power channel 206 necessarily resultsin a loss of power in the n=1 load channel 401. However, the controlleris able to control the switching arrangement 410 to modify the powersourcing for the n=2 load channel 402 such that three of the four phasesof the four-phase load 430 still receive power. The need to power anadditional load channel in the case of a fault in one power channel canbe accounted for by suitably over-rating each of the power channels.

The modified power sourcing in the case of a fault in the r=2 powerchannel 207 is summarised in the final column of Table 1 and can be seento be descriptively equivalent to the r=1 fault case.

Although not included in Table 1, it will be appreciated by thoseskilled in the art that in the event of a load side fault, the circuitbreakers can be opened to protect the associated power source andprevent fault propagation.

Where possible, it also desirable that the connections of and thecontrol of the switching arrangement 410 is such that pairs ofelectrical phases of the load (in this case pairs of phases electricallyseparated by 90 degrees) are powered by the same power channels under‘normal’ operating conditions. For example, referring to Table 1, undernormal operating conditions, the n=1 and n=2 load channels which poweradjacent phases A and B (separated electrically by 90 degrees) are bothpowered by the r=1 power channel. Likewise, the n=3 and n=4 loadchannels which power adjacent phases C and D (separated by 90 degrees)are both powered by the r=2 power channel. The connection of phases toload channels powered by the same power channels results in thecancellation of the second harmonic in the supply current, whichimproves power quality at the power source. This may improve efficiencyand system stability and reduce negative effects such as torque rippleand filter size. Those skilled in the art will appreciate that thecorrect groupings of phases is dependent upon the total number of phasesand the number of pole-pairs in the machine. The load channel pairs inthe example in Table 1 work for an evenly distributed 4-phase, 8-coilelectrical machine with a 6 pole-pair rotor, where load channels n andn+1 are electrically distributed by 90 degrees.

FIG. 5

FIG. 5 illustrates a portion of another electrical power system. Thesystem is the same as the system of FIG. 4, except that in theembodiment of FIG. 5 there are R=3 dc power channels 206-208, a group500 of N=6 dc load channels, a set 520 of N=6 converters, the switchingarrangement 510 includes ten (2N−2=10) contactors, and the poly-phaseload 530 is a six-phase load. As with the embodiment FIG. 4, N=2R.

In the interests of conciseness, insofar as the arrangement andcomponents of FIG. 5 are the same as those of FIG. 4, the description ofFIG. 4 will not be repeated for FIG. 5.

As with the FIG. 4 embodiment, the manner in which the switchingarrangement 510 connects the R=3 dc power channels 206-208 to the N=6load channels 500 is of particular interest.

The following relationship, which relates the load channel index n tothe power channels indices r for which the switching arrangement 510provides connections, is now introduced:

$\begin{matrix}{{r(n)} = \left\{ \begin{matrix}{1,{n = 1}} \\{{{\frac{n}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n}{2}} + 1},{{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {0\ \left( {{mod}2} \right)}}} \\{{\frac{n - 1}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n + 1}{2}},{{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {1\ \left( {{mod}2} \right)}}} \\{R,{n = {N.}}}\end{matrix} \right.} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In other words: the first load channel n=1 is connected to only thefirst power channel r=1; the final load channel n=N is connected to onlythe final power channel r=N; all remaining load channels for which theindex n is even are connected to the (n/2)-th and ((n/2)+1)-th powerchannels; and all remaining load channels for which the index n is oddare connected to the ((n−1)/2)-th and ((n+1)/2)-th power channels.Equation 1 can be utilised for all values of R and N satisfying R<N≤2R,including the embodiment of FIG. 4 (R=2, N=4) and the present embodimentof FIG. 5 (R=3, N=6).

Applying Equation 1 to the present embodiment gives the arrangement ofthe connections between the power channels and load channels, and issummarised in the first two columns of Table 2:

TABLE 2 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 2 1 3 1 and 2 2 2 1 4 2 and3 2 2 or 3 3 5 2 and 3 3 3 3 6 3 3 3 3 Max. Load Channels Per Power 2 33 Channel

A controller (not shown) controls the opening and closing of thecontactors of the switching arrangement 510 such that each of the sixload channels 500 receives power from only one of the three powerchannels 206-208. For the n=1 load channel this is necessarily the r=1power channel 206. For the n=6 load channel this is necessarily the r=3power channel 207. For the n=2, 3, 4 and 5 load channels 402-405,however, one of the two available power channels is selected bycontrolling the opening and closing of the contactors 510. For instance,the n=2 load channel may be powered by the r=1 power channel 206 byclosing the second contactor and opening the third contactor. The n=3load channel may be powered by the r=2 power channel 207 by closing thefifth contactor and opening the fourth contactor.

Under normal operating conditions, with no faults in any of the three dcpower channels 206-208, the controller controls the opening and closingof the contactors so that each of three power channels 206, 207 powersprecisely two of the load channels 500. This is summarised in thirdcolumn of Table 2. Even loading of the power channels 206-208 duringnormal operation is thereby achieved.

In case of a fault condition in which one of the three power channels206-208 is disrupted, the controller controls the opening and closing ofthe contactors of the switching arrangement 510 to reduce the impact onthe downstream load. Specifically, where a load channel was receivingpower from the now-faulted power channel and it is possible to connectthe load channel to another dc power channel, the controller opens andcloses contactors accordingly.

The modified power sourcing in the case of a fault in the r=1 powerchannel 206 is summarised in the fourth column of Table 2. As can beseen, the loss of power in the r=1 power channel 206 necessarily resultsin a loss of power in the n=1 load channel. However, the controller isable to control the switching arrangement 510 to modify the powersourcing for the n=2 load channel such that five of the six phases ofthe six-phase load 530 still receive power. The modified power sourcingin the case of a fault in the r=2 power channel 207 is summarised in thefinal column of Table 2. The response to a fault in the third powerchannel 208 is not shown in Table 2 but will be readily understood inview of the above description.

As explained above with reference to FIG. 4, in order to cancel thesecond harmonic in the supply it is desirable for the power sourcing ofthe load channels to be arranged so that pairs of electrical phases arepowered by the same power channels. In FIG. 5, the number of phases P=6,so pairs of phases electrically separated by 60 degrees are preferablypowered by the same power channel; resulting in a reduction in harmoniccontent. Thus, referring to Table 2, under normal operating conditions,load channels n=1 and n=2 (powered by power channel r=1) power oneadjacent pair of phases; load channels n=3 and n=4 (powered by powerchannel r=2) power another adjacent pair of phases; and so forth. Ifpossible, this approach to power sourcing may also be used following afault in a power channel, but as can be seen from Table 2 it cannot befollowed for all of the phases.

FIG. 6

FIG. 6 illustrates a portion of another electrical power system. Thesystem is the same as the systems of FIGS. 4 and 5, except that in theembodiment of FIG. 6 there are R=4 dc power channels 206-209, a group600 of N=8 dc load channels, a set 620 of N=8 converters, the switchingarrangement 610 includes fourteen (2N−2=14) contactors, and thepoly-phase load 630 is an eight-phase load. As with the embodiments ofFIGS. 4 and 5, N=2R.

In the interests of conciseness, insofar as the arrangement andcomponents of FIG. 6 are the same as those of FIGS. 4 and 5, thedescription will not be repeated.

Applying Equation 1 to the R=4, N=8 embodiment of FIG. 6 gives thearrangement of the connections between the power channels and loadchannels, and is summarised in the first two columns of Table 3:

TABLE 3 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 2 1 3 1 and 2 2 2 1 4 2 and3 2 2 or 3 3 5 2 and 3 3 3 or 2 3 6 3 and 4 3 3 3 or 4 7 3 and 4 4 4 4or 3 8 4 4 4 4 Max. Load Channels Per Power 2 3 3 Channel

Once again, a controller controls the opening and closing of thecontactors of the switching arrangement. Specifically:

i. At all times, each one of the load channels 600 receives power fromonly one of the power channels to which it is connectable by theswitching arrangement 610. This can be seen in all of the columns ofTable 3.

ii. In normal, non-faulted, operation, each power channel 206-209 powersthe same number (two) of load channels. This evenly spreads the load 630amongst the power channels 206-209.

iii. Also in normal, non-faulted operation, where possible, commonlypowered load channels are paired with pairs of phases of the load so asto cancel the harmonics in the supply current.

iv. In case of a fault in one of the power channels 206-209, the loadchannel power sourcing is modified so that load channels are, ifpossible, not connected to the faulted power channel.

v. Preferably, the modification is made holistically such that, afterthe modification, no power channel powers a number of load channelsexceeding three (N/R+1=3).

FIG. 7

In each of the examples described above, the number of load channels Nis equal to twice the number of power channels R, i.e. N=2R. However,the connection arrangement described by Equation 1 can be appliedequally to the case where R<N<2R. FIG. 7 illustrates an example in whichthree load channels 700 are connected to two power channels 206-207 by aswitching arrangement 710.

Applying Equation 1 to the R=2, N=3 embodiment of FIG. 7 gives thearrangement of the connections between the power channels 206, 207 andload channels 700, and is summarised in the first two columns of Table4:

TABLE 4 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 or 2 2 1 3 2 2 2 0 Max.Load Channels Per Power 2 2 2 Channel

Once again, a controller controls the opening and closing of thecontactors of the switching arrangement. Specifically:

i. At all times, each one of the load channels 700 receives power fromonly one of the power channels to which it is connectable by theswitching arrangement 710.

ii. In normal, non-faulted, operation, each power channel 206-207 powersat most two load channels. Since N/R is not an integer where R<N<2R, theload cannot be spread equally between the power channels during normaloperation.

iii. In case of a fault in one of the power channels 206-207, the loadchannel power sourcing is modified so that load channels are, ifpossible, not connected to the faulted power channel.

iv. Preferably, the modification made in response to the fault is madeholistically such that, after the modification, no power channel powersa number of load channels exceeding ┌N/R┐+1. It will be understood that┌N/R┐ is the roofed value of N/R (i.e. the smallest integer greater thanor equal to N/R). It is noted that for the specific case of R=2, N=3, nopower channel powers greater than two load channels (i.e. ┌N/R┐, ratherthan ┌N/R┐+1). For higher values of N this will not be possible.

While the embodiment of FIG. 7 includes N=3 load channels for powering athree-phase electrical load, it may be preferable, regardless of thevalue of R, for N to be at least four (i.e. preferably, N≥4). In thisway, if power is lost in a load channel as a result of either a fault ina power channel or a fault in the load channel, at least threeelectrical phases can remain. In practice three phases may be theminimum desirable number of phases for powering fault-tolerantelectrical loads.

FIG. 8-10

Whilst FIGS. 4-7 show switching arrangements 410-710 with connectionsconforming to Equation 1, alternative connection arrangements arepossible. To this end, FIGS. 8-10 and Tables 5-7 illustrate somealternative switching arrangements. The R=3, N=6 case is used by way ofexample.

First referring to FIG. 8 and Table 5, there is shown a switchingarrangement 810 having ten (2N−2=10) contactors. The contactors of theswitching arrangement 810 connect the first load channel (n=1) to onlythe first power channel (r=1); connect the final load channel (n=N=6) toonly the final power channel (r=R=3); and connect each of the remainingsix (N−2=6) load channels to two different power channels. This issimilar to the arrangement described with reference to FIG. 5 and moregenerally Equation 1. However, the way in which the switchingarrangement 810 connects the intermediate load channels (n=2, 3, 4 and5) to the power channels 206-208 is different than that defined byEquation 1.

TABLE 5 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 2 1 3 2 and 3 2 2 or 3 3 42 and 3 2 3 or 2 3 5 1 and 3 3 3 1 6 3 3 3 3 Max. Load Channels PerPower 2 3 3 Channel

Comparing Table 3 and Table 5, it can be seen from the third columnsthat the normal power sourcing is the same. Furthermore, as can be seenfrom the final rows of the tables, the maximum number of load channelsper power channel is the same when there is a fault in one of the powerchannels. However, it can be seen from the fourth and fifth columns ofthe tables that the required modification to the power sourcing in theevent of a fault in one power channel is different.

Now referring to FIG. 9 and Table 6, there is shown a switchingarrangement 910 having nine (2N−R=9) contactors. The contactors of theswitching arrangement 910 connect the first, second and third loadchannels (n=1, 2 and 3, i.e. the first R load channels) to the first,second and third power channels (r=1, 2, 3) respectively. Each of theremaining three (N−R=3) load channels is connected to two differentpower channels. Thus, unlike the arrangements of FIGS. 4-8 in which onlythe first and final load channels are connected to only one load powerchannel, in FIG. 9 there are R (R=3) load channels connected to only onepower channel.

TABLE 6 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 2 2 2 0 3 3 3 3 3 4 1 and 2 1 2 1 5 2and 3 2 2 or 3 3 6 1 and 3 3 3 1 or 3 Max. Load Channels Per Power 2 3 3Channel

Comparing Table 6 with Tables 3 and 5, it can be seen from the thirdcolumns that, aside from the ordering of the rows, the normal powersourcing is the same. Furthermore, as can be seen from the final rows ofthe tables, the maximum number of load channels per power channel is thesame when there is a fault in one of the power channels. However, it canbe seen from the fourth and fifth columns of the tables the requiredmodification to the power sourcing in the event of a fault in one powerchannel is different.

The connection of a greater number (R) of load channels to only onepower channel further reduces the number of contactors and othercomponents present in the load channels, thus reducing weight andelectrical losses without necessarily compromising on fault tolerance.

Now referring to FIG. 10 and Table 7, there is shown a switchingarrangement 1010 having twelve (2N=12) contactors. The contactors of theswitching arrangement 1010 connect each of the load channels 1000 to twodifferent power channels. Thus, unlike the arrangements of FIGS. 4-9 inwhich some of the load channels are connected to only one power channel,in FIG. the switching arrangement 1010 connects each and every one ofthe load channels 1000 to two different power channels.

TABLE 7 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 and 2 1 2 1 2 1 and 2 2 2 1 3 1 and 3 3 3 1or 3 4 1 and 3 1 3 3 or 1 5 2 and 3 2 2 or 3 3 6 2 and 3 3 3 or 2 3 Max.Load Channels Per Power 2 3 3 Channel

Comparing Table 7 with Tables 3 and 5-6, it can be seen from the thirdcolumns that, aside from the ordering of the rows, the normal powersourcing is the same. Furthermore, as can be seen from the final rows ofthe tables, the maximum number of load channels per power channel is thesame when there is a fault in one of the power channels. However, it canbe seen from the fourth and fifth columns that the required modificationto the power sourcing is different.

The connection of each and every load channel to two different powerchannels increases the component count and thus system weight, andpotentially increases electrical losses. However, in some applicationsthis may be offset by an increase in the ability to rebalance the loadfollowing a fault which may, for example, reduce the harmonic content ofthe current supplied to the load and thus reduce the magnitude ofnegative effects such as torque ripple. Furthermore, this arrangementmay improve the ease of manufacture and assembly, for example where thecontactors and power electronic modules are co-packaged andstandardised.

FIG. 11

In each of the embodiments described above the number of load channels Nsegregated from the R power channels satisfies R<N≤2R. However, thepresent disclosure also provides electrical power systems in which N>2R.To this end, FIG. 11 illustrates an example in which N>2R, specificallyan example in which R=3, N=8.

Whilst other arrangements are possible, in FIG. 11 the switchingarrangement 1110 includes fourteen (2N−2=14) contactors. The contactorsconnect the first load channel (n=1) to only the first power channel(r=1); connect the final load channel (n=N=8) to only the final powerchannel (r=R=3); and connects each of the remaining six (N−2=6) loadchannels to two different power channels. The second, third and fourthload channels are each connected to both the first and second powerchannels. The fifth, six and seventh load channels are each connected toboth the second and third power channels. This is summarised in thefirst and second columns of Table 8:

TABLE 8 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 2 1 3 1 and 2 1 2 1 4 1 and2 2 2 1 5 2 and 3 2 2 or 3 3 6 2 and 3 2 3 3 7 2 and 3 3 3 3 8 3 3 3 3Max. Load Channels Per Power 3 4 4 Channel

As can be seen from the third column of Table 8, during normal operationeach of the power channels provides power to at most three ((i.e.┌N/R┐=3) load channels. This is greater than the at most two (┌N/R┐=2)load channels that must be powered by the power channels in the R<N≤2Rexamples described above.

In the event of a fault in one of the power channels, a controllercontrols the switching arrangement so that each load channels is, ifpossible, not connected to the faulted power channel. The modificationis performed holistically so that number of load channels that a powerchannel must power does not exceed four (La ┌N/R┐+1=4).

From the above description of FIGS. 4-11, it will be appreciated thatvarious fault-tolerant connection arrangements are within the scope ofthe present disclosure. Whilst some of the connection arrangements aredescriptively different, each can be characterized by the followingproperties:

i. N>R. That is, the number of load channels (N) segregated from thepower supply is greater than the number of power channels (R) of thepower supply.

ii. The switching arrangement can connect a number (Q) of the N loadchannels to two different power channels. The number Q is greater thanor equal to one and preferably Q is greater than or equal to N−R.

iii. Except where a load channel is powered by multiple power channels(as described below), each one of the load channels receives power fromat most one of the power channels to which it is connectable by theswitching arrangement. A load channel may receive power from zero powerchannels where all of the connectable power channels are faulted, andpossibly also for load balancing reasons following a fault affectingother load channels.

iv. The switching arrangement and the control of the switchingarrangement is such that in normal, non-faulted, operation, each powerchannel powers at most ┌N/R┐ load channels. Where N is an integermultiple of R, this results in equal sharing of the load between thepower channels. As noted above, ┌N/R┐ is the roofed value of N/R i.e.the smallest integer greater than or equal to N/R.

v. In case of a fault in one of the power channels, a controllercontrols the switching arrangement to modify the load channel powersourcing. The power sourcing is modified so that each load channels is,if possible, not connected to the faulted power channel.

vi. The switching arrangement and the control of the arrangement isholistic such that, in the faulted condition, after the modification tothe power sourcing, no power channel powers a number of load channelsexceeding ┌N/R┐+1.

The above-described switching arrangements and control schemes mayprovide one or more of the following advantages:

a) Increasing the number of independent load channels which power a loadimproves the fault tolerance. For example, a four segregated phase motordrive may run on three phases in the event of a fault. This improvedfault tolerance reduces the need to over-rate the installed powercapacity of the generation and distribution system. However, the faulttolerant electrical systems which provide the R power channels generallyhave a lower optimum number of power channels to reduce complexity,weight and volume. The provision of a system that allows R powerchannels to interface with a larger number N load channels is thusadvantageous because it limits the complexity of the power generationsystem, whilst still allowing for the improved fault tolerance andreduced system over-rating provided by the increased number N of loadchannels.

b) The loss of any single power channel results in the loss of at mostone load channel and thus at most one phase of the poly-phase load. Theloss of electrical and mechanical balance in the load may thus berelatively minor.

c) The above advantage may be achieved with a minimum of additionalloading to the remaining non-faulted power channels. Each remaining,non-faulted, power channel may contribute to the powering of at most oneadditional load channel (i.e. at most ┌N/R┐+1).

d) As mentioned above in point a), the power channels may be over-ratedso that the electrical demands of the poly-phase load(s) and the widerelectrical system can be met in the event of the total loss of one powerchannel. The described connection arrangement and control scheme maycontribute to a reduction in the amount by which the power channels mustbe over-rated to achieve this aim, since any power loss is split amongstall remaining, non-faulted, power channels. Reducing the amount by whichthe power channels must be over-rated may provide improvements in termsof electric machine size and weight, converter size and weight,insulation rating and the like.

e) While the connection of load channels to two different power channelsprincipally provides tolerance against power channel faults, it alsomitigates against faults in the connections between the power channelsand the load channels. A failure in one connection can be overcome byutilising the other connection.

f) The presence of the switching arrangement allows faulted loads oreven faulted phases of a load to be isolated from the remainder of theelectrical power system.

g) Where Q<N, the number of switches in the switching arrangement isreduced, which reduces system weight and complexity. Values of Q=N−2 andQ=N−R may provide particularly good combinations of fault tolerance,relatively low over-rating requirements and weight saving.

In all of the examples described above, load channels are connectable totwo different power channels. It is contemplated that some or all of theload channels could be connectable to three or more power channels.However, the use of additional connections would, whilst improvingfault-tolerance, add to the weight and complexity of the arrangement andthus the use of two connections is generally preferred.

FIG. 12

To further illustrate the advantages associated with the above-describedembodiments, FIG. 12 shows an alternative switching arrangement 1210connecting N=8 load channels with R=4 power channels.

The switching arrangement 1210 of FIG. 12 includes fourteen contactors.The contactors connect the first load channel (n=1) to only the firstpower channel (r=1); connect the final load channel (n=N=8) to only thefinal power channel (r=R=4); and connect each of the remaining six loadchannels to two different power channels. The specific arrangement ofconnections is summarised in Table 9.

TABLE 9 Load Channel Connected Power Power Sourcing (r) (n) Channels (r)Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 2 1 3 2 and 3 2 2 3 4 2 and3 2 2 or 3 3 5 2 and 3 3 3 3 6 2 and 3 3 3 3 7 3 and 4 4 4 4 8 4 4 4 4Max. Load Channels Per Power 2 3 4 Channel

Comparing Table 9 with Table 3, which also has R=4 power channels andN=8 load channels, it can be seen from the third columns that the normalpower sourcing is the same. Furthermore, it can be seen from the fourthcolumns that the modified power sourcing in the presence of a fault inthe r=1 power channel is equivalent in the sense that the maximum numberof load channels per power channel is equal to three (┌N/R┐+1=3).

However, it can be seen from the fifth columns that the modified powersourcing in the presence of a fault in the r=2 power channel is notequivalent: the maximum number of load channels per power channel isequal to three (┌N/R┐+1=3) in Table 3 but is equal to four in Table 9.Thus, if the connection arrangement of FIG. 12 and Table 9 is adopted,the amount by which the power channels must be over-rated to provideequivalent fault-tolerance to the arrangement of FIG. 6 and Table 3 isgreater.

Therefore, whilst the arrangement shown in FIG. 12 may provideimprovements over some alternative arrangements (arrangements withoutany switching arrangement, for example), it may be less preferred thanthe other arrangement described above which may provide greaterfault-tolerance or at least equivalent fault-tolerance with lesscomponent over-rating.

Load Channels Powered by Multiple Power Channels

In each of the embodiments described above, a load channel that isconnectable to two power channels receives electrical power from onlyone of the two power channels. For example, referring to FIG. 4 andTable 1, the n=2 load channel 402 is connectable to both the r=1 powerchannel 206 and the r=2 power channel 207 by the contactors 412, 413,but receives power from only one of the power channels at any one time.In a non-faulted condition, the n=2 load channel 402 receives power fromthe r=1 power channel 206, but in case of a fault in the r=1 powerchannel receives power from the r=2 power channel by closing the thirdcontactor 413.

In an alternative group of embodiments, a load channel (e.g. loadchannel 402) may receive electrical power from both of the connectedpower channels (e.g. power channels 206, 207) in the non-faultcondition. This can be achieved by closing both of associated contactors(e.g. contactors 412, 413). In the case of a fault in one of the twopower channels (e.g. power channel 206), the power sourcing inherentlyswitches to the other, healthy power channel (e.g. power channel 207).

A constraint of this approach is that, since both contactors 412, 413will be closed when a fault in one of the power channels 206, 207occurs, it is necessary for the switching arrangement 410 to provideelectrical segregation between the two power channels 206, 207 toprevent the disruption of one power channel from propagating to thenon-faulted power channel. Suitable segregation may be provided by usingfast acting solid-state circuit breakers to commutate the supply currentfrom the faulted to the healthy power channel, for example based uponundervoltage events on a power channel, whilst maintaining continuity ofsupply to the load channels. This may significantly reduce fault ridethrough capability requirements on the load channels. For load channelside faults, fast acting solid-state circuit breakers will isolate in asimilar capacity breaking to fault currents. In both cases, CurrentLimiting Devices such as Silicon Carbide (SiC) JFETs may be used todynamically limit fault levels.

Whether or not it is possible to provide sufficient segregation willdepend to some extent on the application requirements (e.g. the voltageand power of the system, the cost associated with providing segregationsuitable for the voltage and power, and the relevant safety andcertification requirements of the application), but if it is possiblethen this approach may be preferred. This is partly because thisapproach will inherently improve the fault response time, since nocontactors need to be opened or closed to respond to the fault.

To further illustrate the alternative power sourcing of the presentgroup of embodiments, Table 10 below illustrates the connectionsarrangement and power sourcing for the R=2, N=4 arrangement of FIG. 4.

TABLE 10 Load Channel Connected Power Power Sourcing (r) (n) Channels(r) Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 and 2 2 1 3 1 and 2 1and 2 2 1 4 2 2 2 0 Max. Load Channels Per Power 2 3 3 Channel

As can be seen, the intermediate n=2, n=3 load channels are not onlyconnectable to both the r=1, r=2 power channels, but in normal,non-faulted operation they receive power from both of the two powerchannels. The power sourcing in the case of a fault in one of the twopower channels is the same as in Table 1, though this achieved withouthaving to open or close any contactors because all of the contactors arenormally closed.

Also of note is the maximum number of load channels per power channelunder normal operating conditions. It should be appreciated thatalthough each of the two power channels provides some power to three andnot two load channels, the power supplied to the intermediate loadchannels n=2, n=3 is shared between two power channels and is thus equalto half of one load channel per power channel. The maximum number ofload channels per power channel is thus equal to two, and not three.

A further example is provided in Table 11 below, which summarizes theconnection arrangement and alternative power sourcing when applied tothe N=2, R=3 case of FIG. 7.

TABLE 11 Load Channel Connected Power Power Sourcing (r) (n) Channels(r) Normal Fault 1 Fault 2 1 1 1 0 1 2 1 and 2 1 and 2 2 1 3 2 2 2 0Max. Load Channels Per Power 2 2 2 Channel

Comparing Table 11 with Table 4, it can be seen that, using thealternative power sourcing, the n=2 load channel is powered by both ther=1 and r=2 power channels under normal operating conditions. Thus, eventhough the ratio N/R is not equal to an integer, the load can be spreadevenly between the power channels.

It will be appreciated that although the alternative power sourcingtechnique has only been described with reference to FIGS. 4 and 7, itcould equally be applied to any of the other embodiments describedabove. The control schemes—particularly the holistic control of theswitching arrangement to limit the power channel loading to ┌N/R┐+1 loadchannels per power channel—applies equally to this group of embodiments.

Furthermore, it could be applied to some but not all of the loadchannels connectable to two different power channels. For example, inTable 10, the n=2 load channel may receive power from both the r=1 andr=2 power channels during normal operation, whereas the n=3 load channelmay receive power from only one of the two power channels during normaloperation.

FIGS. 13A-13B

The reconfigurable switching arrangements 410-1210 described aboveprovide fault-tolerance in case of a disruption to one or more of thepower channels that supply power to the load channels. Furthermore, incase of a fault in a load, the switching arrangements 410-1210 allow theentire load or a subset of the load channels or phases to be isolatedfrom the remainder of the electrical power system. However, theswitching arrangements 410-1210 may not provide completefault-tolerance, and in particular may not always prevent faults ortransient events in one load from disrupting the power channels and theother loads that are connected to the power channels.

For instance, even if a faulted load can be isolated, the faulted loadcould draw an excessive current from the power channels in the timebetween the fault arising and the isolation of the load. This couldstarve other, non-faulted, loads of power or expose the other loads tovoltage dips. As another example, a transient event caused by a loadwill generally not require the load to be isolated, but could causeover-voltage events that propagate to other loads connected to the powerchannels.

To address these problems, the load channels may, optionally, includecurrent limiting devices (CLDs). The CLDs may be incorporated into, orbe connected upstream or downstream from, the contactors of theswitching arrangements.

A CLD is a component with a variable impedance that acts to limit themagnitude of the current flow, for example in response to an upstream ordownstream fault or transient event. FIG. 13A illustrates how each ofthe contactors or circuit breakers (e.g. the contactors 411) shown inthe systems of FIGS. 4-12 may include not only a contactor 450 but also,connected in series with the contactor 450, a CLD 460. The CLD 460 maybe unidirectional or bidirectional. For example the dc contactor may beconnected in series with a single unidirectional CLD 460 (left handdrawing in FIG. 13A) or a bidirectional CLD formed from a pair ofunidirectional CLDs 461, 462 connected in anti-parallel.

The CLD 460 may be a passive component such as an inductor; an activecomponent such as a current-limiting diode; or another component such asa superconducting fault current limiter. By way of example, FIG. 13Billustrates a design for a current-limiting diode 460 (sometimesreferred to as a constant-current diode), which includes an n-channelJFET whose gate terminal is shorted to the source terminal with aninterconnecting resistor.

FIG. 14A

FIG. 14A illustrates how the use of a CLD connected in a load channelcan improve the fault-tolerance of an electrical power system against afault arising in an electrical load powered by the load channel.

The top-most curve of FIG. 14A shows how the current drawn by a loadchanges when a fault arises in the load. The middle curve shows thechange in the voltage experienced by another, non-faulted, load that isconnected to the same power channel by another load channel. Thebottom-most curve shows how the dynamic impedance of a CLD connected tothe faulted load changes when the fault arises. In the top-most curveand middle curve, solid lines represent the system response when no CLDis present. The dotted lines represent the system response when the CLDis present.

First considering the system without the CLD, it can be seen that whenthe load fault arises, the current drawn by the load from the electricalgenerator and power channel rises rapidly. In response to this, there isa voltage drop on the power channel and thus the other, non-faulted loadexperiences a sudden and significant under-voltage event. Theunder-voltage event may be such that the non-faulted load cannot drawsufficient electrical power from the power channel until the fault isresolved or isolated. This may be particularly problematic if thenon-faulted load has a critical function. For example, if thenon-faulted load is a propulsive load or a fuel pump, an associatedaircraft may experience a temporary loss of thrust. As another example,if the non-faulted load is an oil pump or coolant pump, engine oraircraft components may be damaged or suffer increased wear if there isa temporary loss of function.

Now considering the system with the CLD, when the load fault arises, thecurrent drawn by the load from the electrical generator and powerchannels begins to rise. In response to this rise, the impedance of theCLD rises rapidly to curtail the rise in the current drawn by thefaulted load. Thus, the rise in the current drawn by the faulted loadquickly levels off and the associated voltage drop experienced by thenon-faulted load is minor and does not affect its function.

FIG. 14B

FIG. 14B illustrates how the use of a CLD connected in a load channelcan improve the response of an electrical power system when a transientover-voltage event occurs in a load. The bottom curve shows how thedynamic impedance of a CLD connected to the load that experiences thetransient event changes in response to the transient. The top curveshows how the voltage level experienced by another load changes when thetransient event occurs. In the top curve, the solid line represents theresponse when no CLD is present whilst the dotted line represents theresponse the CLD is present.

First considering the response where no CLD is present, the over-voltagetransient rapidly propagates to the other load since the other load isconnected to the same power channel. This may result in the other loadelectrically failing due to electrical breakdown, or suddenly drawing amuch larger amount of electrical power, which could damage the load orat least temporarily affect its function. Once again, this may be aparticular problem if the other load has a critical function.

Now considering the system with the CLD, the CLD reacts to the transientevent by rapidly increasing its impedance. This rapid increase inimpedance limits the resulting increase in the over-voltage and thusprotects the other load from the transient event.

Thus, it will be understood that by combining the switching arrangementsdescribed with reference to FIGS. 4-12 with CLDs in the load channels, areconfigurable and highly fault-tolerant electrical power system isachieved. Specifically, no single fault in a power channel can affectmore than one load channel per group of load channels, and no fault inone load channel can disrupt other load channels and indeed other loads.In combination with a multi-redundant generator system such as the onesdescribed with reference to FIGS. 2-3, an electrical power anddistribution system is provided in which a wide array of faults can bemanaged and in which no single fault can have a significant impact onthe overall system.

Whilst the present embodiments have been described with respect to gasturbine engines, it is contemplated that the electrical power systemconfigurations described herein may be extended to facilitate connectionof rotary electric machines with other types of rotating machinery. Forexample, the rotary electric machines may be connected with other typesof heat engines, for example internal combustion engines such asreciprocating or Wankel-type engines. Other types of heat engines suchas steam turbines operating according to the Rankine cycle may beconnected. Combinations of different types of rotating machinery may beconnected.

Thus it will be appreciated that the invention described herein may beexpressed as an electrical system for connecting rotary electricmachines with other rotating machines, the system having the propertiesand attributes described above.

It should also be noted that whilst the present embodiments have beendescribed with reference to a turbofan engine 101 for an aircraft, itwill be understood that the principles of the described electricalsystems may be applied to other installations, for example in a marineenvironment such as on a naval vessel powered by gas turbines, or in anenergy production environment such as in a power station utilisingnatural gas fired gas turbines, or any other suitable application.

Furthermore, it should be appreciated that whilst the presentembodiments have been described with reference to arrangements in whichrotary electric machines and an associated set of ac-dc convertercircuits output a number R of dc power channels, the R dc power channelsmay instead be output by an R-channel dc power source, for example anR-channel battery or arrangement of fuel cells. Such an arrangementcould, for example, be used in a hybrid-electric or purely electricaircraft having batteries and/or fuel cells configured to output R dcpower channels. The n dc load channels may power propulsive ornon-propulsive loads.

Various examples have been described, each of which feature variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features andthe invention extends to and includes all combinations andsub-combinations of one or more features described herein.

1. An electrical power system for distributing electrical power in an arrangement comprising one or more gas turbine engines, the electrical power system comprising: one or more rotary electric machines, each mechanically coupled to a gas turbine engine spool; a set of converter circuits connected to the one or more rotary electric machines for conversion between alternating current (ac) and direct current (dc), wherein the one or more rotary electric machines and the set of converter circuits are arranged to output a number R≥2 of dc power channels, each dc power channel having a respective index r=(1, . . . , R); and a group of N dc load channels connected to the R dc power channels by a switching arrangement, wherein each dc load channel has a respective index n=(1, . . . , N) and R<N≤2R; wherein the switching arrangement is operable to connect the r-th power channel to the n-th load channel according to the relationship: ${r(n)} = \left\{ \begin{matrix} {1,} & {n = 1} \\ {{{\frac{n}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n}{2}} + 1},} & {{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {0\ \left( {{mod}2} \right)}} \\ {{\frac{n - 1}{2}\mspace{14mu}{and}\mspace{14mu}\frac{n + 1}{2}},} & {{1 < n < {N\mspace{14mu}{and}\mspace{14mu} n}} = {1\ \left( {{mod}2} \right)}} \\ {R,} & {n = {N.}} \end{matrix} \right.$
 2. The electrical power system of claim 1, in which N=2R.
 3. The electrical power system of claim 1, in which R=2, 3 or
 4. 4. The electrical power system of claim 1, in which N≥4.
 5. The electrical power system of claim 1, further comprising a controller configured to control the operation of the switching arrangement.
 6. The electrical power system of claim 5, in which the controller configured to control the operation of the switching arrangement so that, at all times, each one of the N−2 load channels for which 1<n<N receives power from only one of the two power channels to which it can be connected by the switching arrangement.
 7. The electrical power system of claim 1, in which, in a no fault condition, each one of the N−2 load channels for which 1<n<N receives power from both of the two power channels to which it is connected by the switching arrangement.
 8. The electrical power system of claim 1, comprising a controller configured to control the operation of the switching arrangement so that, in a fault condition in which one of the R power channels has a fault, each one of the respective N−2 load channels for which 1<n<N receives power from a non-faulted power channel of the two power channels to which the respective load channel can be connected by the switching arrangement.
 9. The electrical power system of claim 1, comprising a controller configured to control the operation of the switching arrangement so that, at all times, each one of the R power channels delivers electrical power to a number of load channels not exceeding ┌N/R┐+1.
 10. The electrical power system of claim 1, comprising a controller configured to control the operation of the switching arrangement so that, in a no fault condition, each one of the R power channels delivers electrical power to a number of load channels not exceeding ┌N/R┐.
 11. The electrical power system of claim 1, comprising a controller configured to control the operation of the switching arrangement so that, in a fault condition in which one of the R power channels has a fault, each one of the R power channels delivers electrical power to a number of load channels not exceeding ┌N/R┐+1.
 12. The electrical power system of claim 1, in which the N dc load channels are connected to and power a poly-phase electrical load, and in which the controller is configured to control the switching arrangement so that, in a no fault condition, pairs of phases of the poly-phase electrical load are powered by the same power channels so as to cancel harmonics in current supplied by the power channels.
 13. The electrical power system of claim 12, in which the two phases of each pair of phases are electrically separated by 360/P degrees, P being a total number of phases of the poly-phase electrical load.
 14. The electrical power system of claim 1, in which, for each respective load channel of a plurality of the N load channels, the switching arrangement includes or is connected to a current limiting device (CLD) operable to limit an amount of current flowing from the power channels to a load connected to the electrical power system via the respective load channel.
 15. The electrical power system of claim 1, in which the N load channels are connected to one or more loads including at least one of: an electrically driven fuel pump; an electrically driven oil pump; an electrically driven cabin blower.
 16. A gas turbine engine comprising the electrical power system of claim 1, wherein the one or more rotary electric machines of the electrical power system are mechanically coupled to one or more spools of the gas turbine engine.
 17. An arrangement comprising: a first gas turbine engine having a spool; a second gas turbine engine having a spool; and the electrical power system of claim 1, in which the one or more rotary electric machines includes a first rotary electric machine mechanically coupled to the spool of the first gas turbine engine and a second rotary electric machine mechanically coupled to the spool of the second gas turbine engine.
 18. An aircraft propulsion system comprising the electrical power system of claim
 1. 19. The aircraft propulsion system of claim 18, comprising: one or more gas turbine engines, wherein the one or more rotary electric machines of the electrical power system are mechanically coupled to one or more spools of the one or more gas turbine engines; and one or more propulsors, wherein the R power channels of the electrical power system are arranged to deliver power to the one or more propulsors, and the N load channels are electrically connected to one or more non-propulsive loads.
 20. An aircraft comprising the electrical power system of claim
 1. 